1. Field of the Invention
The present invention relates to high-value polyurethane (PU) elastomers and high-value polyurethane-urea elastomers which are particularly easy to process as a result of use of low-viscosity prepolymers based on naphthalene 1,5-diisocyante, and also to a process for production of these.
2. Description of Related Art
PU cast elastomers are valuable materials, and are produced from polyisocyanates, polyols, and chain extenders, and/or crosslinking agents. An industrially important polyisocyanate here is naphthalene 1,5-diisocyanate (NDI).
Polyols that can be used are the mostly hydroxy-group-terminated polyester or polyether polyols that are known to the person skilled in the art. Chain extenders used are short-chain polyols, e.g. ethylene glycol or butylene 1,4-glycol, and likewise short-chain polyamines, e.g. 4,4′-methylenebis(2-chloroaniline) (MBOCA), or else water.
Polyurethanes based on naphthalene 1,5-diisocyanate (e.g. Desmodur® 15 from Bayer MaterialScience AG) have been known for a long time, and are marketed by Bayer MaterialScience AG with trademark Vulkollan®, preferably in high-value applications. A feature of polyurethanes based on naphthalene 1,5-diisocyanate here is that they have not only excellent dynamic mechanical properties but also exceptional long-term usage performance in comparison with many other materials.
The use of NDI for the production of cast elastomers is likewise known, and is described by way of example in DE-A 1694169. Here, polyester polyols having softening points in the range from 60 to 145° C. are used.
In the production of PU cast elastomers, a distinction is made between the one-shot process in which the reactants polyisocyanate, polyol, and chain extender are reacted simultaneously with one another and the prepolymer process in which the polyol is reacted with the polyisocyanate and only then is the chain-extender component used.
Oligomers formed from NDI and from low-molecular-weight chain extender have poor solubility in the reacting melt made of NDI, chain extender, and polyol, and therefore precipitate and are unavailable for complete reaction and coupling to the polyol component, and for this reason elastomers based on NDI, in contrast to elastomers based on hexamethylene diisocyanate, toluene diisocyanate, or diphenylmethane 4,4′-diisocyanate, are produced only by the prepolymer process, and not by the one-shot process.
On the other hand, because of the comparatively high melting point of NDI, 127° C., it is necessary to produce the NCO prepolymers based on NDI at reaction temperatures above that temperature, in order to ensure that the reaction mixture is homogeneous. However, the NCO prepolymers based on NDI that are widely used for the production of NDI-based cast elastomers have only a short shelf life when exposed to high temperature (≧127° C.), since the high temperatures cause side-reactions, the final result of which is a reduced NCO index, in conjunction with a viscosity rise, and processing therefore at least becomes more difficult, if not impossible.
The above problems with conventional NDI prepolymers having short shelf life form the background to recommended processing methods which require that the chain-extension reaction be carried out within 30 minutes after production of the NDI prepolymer, and to descriptions in the literature which cast doubt in very general terms on the storage capability of NDI prepolymers: in “Solid Polyurethane Elastomers”, P. Wright and A. P. C. Cummings, Maclaren and Sons, London 1969, pp. 104 ff. in chapter 6.2. we find the following:    “6.2.1. Unstable Prepolymer Systems (Vulkollan®) (Vulkollan®; Trademark for Cast Elastomer Systems Based on Naphthalene Diisocyanate (NDI) from Bayer MaterialScience AG).
Vulkollan® is manufactured by a prepolymer route, although the prepolymer is non-storable and must be further reacted within a short interval of time. The prepolymer so formed is relatively unstable since further undesirable side reactions can take place. To reduce the possibility of these side reactions occurring, the next stage in the process, viz. the chain extension, should take place as soon as possible but within a maximum of 30 minutes.”
These statements clarify the reason for the importance of low initial viscosity of the prepolymer, since even within the recommended 30 min the viscosity rises markedly, and therefore processing, in particular of relatively large and/or relatively complicated moldings, is made at least more difficult, if not impossible, since the reacting melt made of high-viscosity prepolymer and chain extender then fails by way of example to reach mold regions that are distant from the gate. A low initial viscosity of the NDI prepolymers is generally also desirable in order to ensure that it is easy to disperse the chain extender within the material. In particular NDI-based elastomers with high hardness values, based on prepolymer with high NCO content, where by way of example 1,4-butanediol is used as chain extender, also exhibit extremely low casting times, and again this makes it very difficult to produce complicated moldings.
WO 2007/022884 A1 describes 2,2′-thiodiethanol as chain extender permitting achievement of relatively long casting times for the reaction of PU-NCO prepolymers with crosslinking agents, where the mechanical properties of the resultant PU elastomers are no poorer than those of comparable prior-art elastomers. This longer casting time permits production of complicated moldings made of hard PU elastomers.